The beauty of a gemstone, such as a cut diamond, derives from its light handling ability. What attracts the eye is the “game of light” played by a well-cut diamond as incident light is reflected and refracted off its many facets. Diamonds and other gemstones may be cut according to many different standardised cut patterns such as the standard round brilliant cut, oval, pear, marquise, radiant, princess, heart, emerald cut etc. The most popular cut is the standard round brilliant (SRB) cut as shown in FIG. 1. Diamond cutting and polishing is a highly skilled art and a well-cut diamond, having superior optical performance, will command a significant price premium over a poorly cut diamond having inferior optical performance.
When showing a cut diamond to an untrained observer, one frequently refers to the four C's of a gemstone, being its carat weight, its clarity, its cut and its colour. Carat weight, clarity and colour can be relatively easily measured objectively and are therefore generally useful. Cut may also be specified or measured in terms of the geometry of the various facets. However, what really matters is the light handling ability of the gemstone and reference is often made to more subjective parameters of a cut diamond, such as its brilliance (the intensity of light returned), scintillation (fast and local fluctuations in the light returned as the diamond moves relative to the lighting conditions), fire (the dispersion of white light into spectral colours) and symmetry (the symmetry of light patterns such as the so-called “hearts and arrows”). It can be difficult, particularly for an untrained observer, to make a personal appraisal of these subjective parameters. It can also be difficult to compare the light handling abilities of two diamonds with the same carat weight, clarity and colour, and therefore to appreciate why one is more valuable than the other.
It is recognised that properties such as brilliance, fire and symmetry should be derived, whether by experimental observation or theoretical computer modelling, under a variety of lighting conditions. Moreover, with the property of scintillation it is clearly essential to observe or model the diamond under a variety of lighting conditions. Electronic apparatus for capturing images of gemstones under varying lighting conditions for analysis are known.